1. Field of Application
The present invention relates to a control apparatus for controlling a power converter which incorporates switching elements and is connected to a rotary machine operable as a drive motor of a vehicle, and for controlling a cooling apparatus to effect cooling of the rotary machine and the switching elements of the power converter. In particular, the invention relates to a control apparatus for a system in which the cooling apparatus circulates a coolant fluid via a path through the rotary machine and the power converter.
2. Background Technology
Types of control apparatus are known for controlling a rotary machine such as a motor/generator which is installed in a vehicle and serves as a drive motor for the vehicle when operated as an electric motor. Such a control apparatus includes a power converter incorporating switching elements, which is electrically connected for supplying/receiving power to/from the rotary machine, and a cooling apparatus for cooling the rotary machine and the power converter, preventing excessive temperature increase of the switching elements. The cooling apparatus may effect the cooling by circulation of a coolant fluid via a path through the rotary machine and the power converter. Other methods of preventing excessive temperature rise of the switching elements in such a system have been proposed. A control apparatus is described in Japanese patent first publication No. 2006-211886 (referred to in the following as reference 1) for example, for application to a 3-phase AC synchronous electric motor which functions as the drive motor of a vehicle. A DC supply voltage is converted to a 3-phase AC supply voltage by on/off switching at a high switching frequency (carrier frequency) by the switching elements, with the switching being modulated for controlling the AC supply frequency (i.e., controlling the motor speed). The switching is further modulated for controlling the level of electrical power supplied from the inverter (thereby determining the level of current passed by the switching elements) in accordance with a command value of torque expressing a torque demand. With the system described in reference 1, the switching frequency is increased in successive steps, for successively increasing ranges of motor speed. For each switching frequency, a predetermined relationship is established between the detected temperature of the switching elements and allowed maximum values of the torque demand, as illustrated in FIG. 3 of reference 1 (showing values of switching element temperature and of torque demand along the horizontal and vertical axes respectively). The relationship is such that the maximum permitted value of the torque demand becomes increasingly limited (is progressively reduced below 100%) as the detected switching element temperature increases. The maximum level of supplied AC power is limited accordingly. The switching elements can thereby be prevented from reaching an excessively high temperature due to heating caused by switching losses.
However with such a technique, the effects of limiting the torque developed by the motor when providing motive power to a vehicle may present problems. Specifically, when the maximum available torque is limited to a relatively low value, torque restriction may be frequently applied, for periods of long duration. Hence the effective level of torque may be insufficient and the user may have feelings of unease.